Radiation-induced gastrointestinal syndrome (RIGS) results from a combination of direct cytocidal effects on intestinal crypt and endothelial cells and subsequent loss of the mucosal barrier, resulting in microbial infection, septic shock and systemic infiammatory response syndrome. Currentiy, there is no therapy for RIGS. Irradiation induces apoptosis of crypt endothelial cells, intestinal stem cells (ISC) and enterocytes within hours. We rationalized that the acute loss of cells in situ requires rapid compensation of their functions and this was best achieved with cell replacement therapies, e.g., blood transfusion for hemorrhage. The stroma of solid organs contains a variety of supporting cells, such as, mesenchymal and microvascular endothelial cells, macrophages and lymphocytes. These stromal cells provide the niche and could supply critical growth factor/signals for ISC regeneration. For example, upon intestinal mucosal disruption, resident macrophages in the intestinal submocosal layers are activated by pathogen-derived ligands for Toll-like receptors (TLR) and transmit regenerative signals to ISCs. We thereby propose intestinal regenerative therapy with a combination of systemic administration of growth factors and cell replacement therapy to salvage Gl function post-radiation exposure. In order to develop an stem cell-based therapeutic strategy for RIGS, we hypothesized that combinations of: a) intestinal stem cell growth factor, R-spondinl (R-spol), b) TLR ligands, and c) transplantation of bone marrow-derived endothelial progenitor cells (EPC) and mesenchymal stem cells (MSC) would restore the IR-damaged ISC niche, protect against IR-induced cell death and provide growth signals for host ISC regeneration, thus providing protection and mitigation from RIGS. Aim I Pathophysiologic Mechanisms, Discovery and Validation of Molecular Targets in RIGS Aim II will investigate whether acceleration of ISC regeneration could mitigate/protect RIGS by administration of a Wnt agonist, R-spondinl and a BMP antagonist. Aim III will examine whether repair of the ISC niche by TLR activation and/or bone marrow-derived adherent stromal cell-based therapies could mitigate RIGS in mice. The final goal is to identify radio-mitigating factors secreted by stromal cells